To digitize sensor outputs from 0 V to VRef, designers once had to run the signal-conditioning amplifier and analog-to-digital converter (ADC) on positive and negative supplies. There were several problems with that. For example, the driver-amp output transistors required several tens of millivolts to maintain linearity, and the negative supply would dissipate power. Also, the need for a negative supply introduced problems in terms of circuit board routing and bill-of-materials cost.
In the past, solving these problems meant eliminating the negative supply and reducing the level of the input signal. Unfortunately, that meant that the analog-to-digital converter (ADC) might not be able to generate codes near the top and bottom of the transfer function.
To avoid those problems, Linear Technology has added a novel feature to its LTC2379-18 (see the figure) successive approximation register (SAR) 18-bit ADC. Linear calls it Digital Gain Compression. With Digital Gain Compression enabled, the ADC scales the input in a way that maps zero-scale code to 0.1 VRef and full-scale code to 0.9 VRef.
For a reference voltage of 5 V, then, the input range will actually be from 0.5 to 4.5 V, providing headroom for the drive amplifier so it doesn’t need a negative supply. That way, the system designer gets the full 262,144 codes of the 18-bit ADC, and they cover the entire sensor range.
The LTC2379-18 samples at 1.6 Msamples/s and its signal to noise ratio (SNR) is specified at 101 dB, which Linear says is 4 dB better than the chip’s closest competition. Guaranteed maximum integral non-linearity (INL) is ±2 least significant bits (LSBs) with no missing codes. Operating from a 2.5-V supply, the ADC consumes 18 mW. True no-latency operation enables accurate one-shot measurements even after lengthy idle periods with no minimum sample rate required.
Although the LTC2379-18 is the highest-resolution member of Linear’s new family of SAR ADCs for industrial applications, it is not the fastest. There is also the 16-bit, 2-Msample/s LTC2380-16, with 96-dB SNR and ±0.5-LSB maximum integral non-linearity (INL). These devices will be followed by more 18/16-bit high-performance SAR ADCs with speeds ranging from 250 ksamaples/s up to 2 Msamples/s, all with digital gain compression.
The LTC2379-18 costs $29.95 and the LTC2380-16 costs $24.50, both in 1000-piece quantities.